Acknowledgement

The following information is taken from Hot Mix Asphalt Technology - March/April 2010 edition, Asphalt's Stake in Railroading: Asphalt is a proven winner for creating track beds that provide a safe base for high speed passenger trains or heavy freight service, by Dr. Jerry Rose, P.E.

Railroad Trackbeds

While most track beds is the U.S. use the ballasted trackbed design, newer designs using asphalt are considered for high-speed rail and for heavy freight rail.

Two different roadbed designs employ asphalt layers. The most common design in the U.S. is known as underlayment, in which the asphalt layer is placed directly on select subgrade or old roadbed. A layer of ballast is placed on the asphalt on which the track is positioned. This changes little from the normal trackbed design, since the asphalt layer merely replaces the granular sub-ballast layer. The asphalt layer is similar to a paved lane of highway, except it is contained within the track structure to form a structural hardpan layer between the ballast and the subgrade or existing roadbed.

The overlayment design is a ballast-less trackbed and involves placing the asphalt layers in a similar manner, except no ballasts is used between the asphalt and the railroad ties. The ties are placed directly on the asphalt surface. Cribbing aggregate is then placed between the ties and at the end of the ties to restrain track movement. This design is used to an extent in Europe to support direct fixation concrete slab track or two-block concrete tie track.

Benefits of Asphalt Underlayment

The benefits of asphalt underlayment trackbeds have been well documented by research and observations over the past 30 years. Among those benefits:

An asphalt underlayment provides a strengthened track support layer below the ballast that uniformly distributes and reduces pressures to within the bearing capacity of the roadbed or subgrade.

As a waterproof layer, it provides containment for the materials below it in the roadbed, improving the stability and load-carrying capacity capability of track structures – even on roadbeds of marginal quality.

The asphalt underlayment provides a consistently high level of confinement for the ballast above it, enhancing shear strength and uniform pressure distribution.

Asphalt underlayments provide a resilient layer between the ballast and the roadbed to reduce the likelihood of subgrade pumping without substantially increasing track stiffness.

The asphalt underlayment is an all-weather, uniformly stable surface on which to place the ballast and track superstructure.

Applicable for Heavy Freight and High Speed

Asphalt underlayment delivers the greatest benefit to railways carrying heavy freight traffic or high-speed passenger traffic. The asphalt layer can correct any of the following conditions:

Difficulty in establishing and maintaining a roadbed with sufficient strength and stability to support the wheel loadings on the ballast and track.

Difficulty in establishing and maintaining proper drainage to convey surface water away from the track structure.

Difficulty in preventing ground water from weakening the track structure.

Abnormally high impact stresses at joints, bridge and tunnel approaches, other special track works, or open track where track stiffness varies abruptly.

Areas where these conditions exist are likely to show rapid ballast contamination, excessive wear of track components, and below-standard track geometric parameters. Maintenance costs become too expensive to continue safe line-speed operations so "slow-orders" must be imposed, which reduce operating efficiency.

Asphalt Mix Design

Recommended asphalt mix specification, trackbed section design, and application practices have evolved over the years. Slight variation from the initial mix designs and construction techniques are typical and have not affected trackbed performance.

The asphalt mix that has the best properties for the track structure environment is a low to medium modulus (plastic) mix, having design air voids of 1 to 3 percent. The mix will easily compact to less than 5 percent air voids in place. A local dense-graded highway base mix with a maximum aggregate size of 1.0 to 1.5 inches is typically specified.

Ideally, the asphalt binder content can be increased by about 0.5 percent above optimum for highway applications because rutting and bleeding are not concerns in the insulated trackbed environment. This is similar to the bottom, or fatigue-resistant asphalt layer of the Perpetual Pavement system being discussed for highway pavements in the U.S.

Typical Width and Thickness

The typical asphalt layer width is 12 feet for open track, but is placed wider under special trackwork, such as turnouts, to provide support under the longer ties.

The thickness of the asphalt layer depends on the quality of the roadbed's subgrade support and traffic loadings. A 6-inch thick layer is normally used for average conditions. For unusually poor roadbed support conditions, and for high-impact areas, a minimum of 8-inches is used. Ballast thickness normally ranges from 8 to 12-inches. A 6-inch-thick asphalt layer that is 12-feet wide requires 0.42 tons of asphalt per track foot.

The asphalt layer should be extend a reasonable length beyond the ends of the special trackwork so that subsequent track surfacing operations and any impact from track stiffness changes will not infringe in the area.

The roadbed should be reasonably well compacted, well-drained, and capable of accommodating the hauling and spreading equipment without excessive rutting or deformation. A slight crown or side slope is desirable. Sub-surface drainage or roadbed support improvements can be implemented prior to placing the asphalt if site conditions warrant it.

Structural Design

The structural design of railway trackbeds containing asphalt underlayment can be performed using KENTTRACK, a finite element computer program. The primary governing factor is limiting the vertical compressive stresses, or permanent deformation, on the subgrade.

For the asphalt layer, the tensile strains at the bottom of the asphalt control its service life. Damage analyses are conducted and used to predict the service life of the tracked components for various combinations of traffic, tonnages, subgrade support, and component layer compositions and thicknesses.